The Neuropathology Research Laboratory under the direction of Dr. DeArmond has performed a service function through a series of Neuropathology Cores (NP Cores) as well as a hypothesis testing function in studies of prion diseases since 1984 in collaboration with Stanley Prusiner and colleagues. The NP Cores have contributed directly to more than 75 prion disease-related publications in that time. This NP Core will perform a full range of neuropathological and immunohistochemical services including: perfusion of animals when needed; dissection of brain tissues as needed; snap freezing brain and other tissues as needed; standard neurohistological stains of formalin-fixed, paraffin-embedded tissue sections; histochemical stains of crystal sections of muscle; plastic embedding and toluidine blue staining of thick sections of peripheral nerve; electron microscopy; and full autopsies as needed. The NP Core has also pioneered developed of techniques to identify and quantify PrPc and PrPSc in tissue sections. The two standard techniques we use today are: the hydrolytic autoclaving method to eliminated PrPc and localize PrPSc in formalin-fixed, paraffin-embedded sections and the histoblot technique that can localized and quantify either PrPc or PrP in tissue sections. The two standard techniques we use today are: the hydrolytic autoclaving method to eliminate PrPc and localize PrPSc in formalin- fixed, paraffin-embedded sections and the histoblot technique that can localize and quantify either PrPc or PrPSc in unfixed, frozen tissue sections. The NP Core continues to test new PrP antibodies by these two immunohistochemical techniques. This core also maintains a bank of paraffin-embedded and frozen tissues. The NP Core continues to test new PrP antibodies by these two immunohistochemical techniques. This core also maintains a bank of paraffin embedded and frozen tissue. The NP Core maintains a now massive database of over 8000 animals for which either neuropathological or histoblot analysis has been performed. The NP Core helps Project Leaders interpret the neuropathological/immunohistochemical findings in the context of their hypotheses and questions, quantifies neuropathological and immunohistochemical results as needed, and prepares both block and white or color publication and notebook quality reproductions. Finally, the projects in this Grant are now requiring accurate quantification of nerve cell and synapse numbers. To this end, computer-assisted stereological techniques are being added to this core.